This invention relates generally to structures which form an air seal in gas turbine components and the like. More particularly, the present invention relates to a process for repairing damaged and corroded knife edge air seals of a turbine.
Turbine blades of gas turbines to which the invention is adapted employ a multiplicity of blades mounted around the peripheral portion of a rotatable disk. The disk rotates within an enclosed duct of a gas turbine under the drive forces of gases which have a high pressure and a high temperature. Air seals are required to seal between the ducts and the circumferential rings formed by mating shrouds of the turbine blades to seal the gases in the turbines. The knife edge components which form the turbine air seals are relatively thin, typically on the order of 0.018 to 0.020 inches thick, and are typically formed from nickel based super alloys.
High temperature gases which pass over the turbine air seals cause the turbine air seals to wear or corrode. The thermal effects of combustion gas contaminants, the thermal stresses imposed by the cyclic nature of the operation of the turbine and the frictional engagement of the rotating blade tip against the stationary turbine outer air seal are some of the factors which cause damage to the air seals over time. A principal manifestation of the damage is an increase in the clearance gap between the seals and the blade tips. The increase in the clearance gap decreases the turbine efficiencies due to the power loss from escaping gases. In addition, turbine components which are heated by the escaping hot gases tend to deteriorate. A significant segment of turbine technology is directed to the repair of turbine air seals and the maintenance of air seal integrity.
Conventional turbine air seal repair technology in one approach involves the machining of portions of the corroded damaged knife edge portions and the application of a weld build-up of the worn seal edges. The weldment is then remachined to the original specifications of the turbine component. However, considerable design and tooling efforts are frequently expended in controlling the distortion inherent in the welding process which causes some weld metal shrinkage and part distortion. Additional tooling is frequently provided to correct distortion where shrinkage otherwise cannot be controlled. The design and tooling requirements for controlling distortion may result in a repair process which is not cost effective in comparison with the cost of new replacement parts.
Arrigoni, U.S. Pat. No. 4,285,108 discloses a method wherein a damaged turbine air seal is removed by an abrasive belt to create a stub-like structure. A piece of material is joined to the stub such as by welding or brazing. The air seal weldment is then finished by an abrasive belting apparatus. DeMusis, U.S. Pat. No. 4,028,788 discloses a method wherein a bead of welding material is added to opposite side portions and end portions of an air seal flange to build up the worn areas. The built-up flange materials are then removed to reshape the air seal flange. McDonald et al, U.S. Pat. No. 3,771,978 discloses a method for repairing the knife edge fin wherein a knife edge is honed and a groove is reamed in the upper portion of the blade. A metallic wedge is mounted in the groove and secured by a thermal fitting process. The external circumferential edge is then sharpened to form the knife edge of the air seal.